Field-effect transistors (FET) has been considered to be an ideal technology for nonvolatile memory because of its random access, high speed, low power, high density and simplicity. For a nonvolatile semiconductor storage device in which each memory cell is composed of an FET provided with a floating gate covered by an insulating film and used as a charge storing layer, data is stored by controlling the amount of electrons stored in the floating gate thereby changing the threshold voltage of the transistor. When programming or erasing data into or from the memory cell, electrons are either injected or ejected from the floating gate via the insulating film.
Electron injection/ejection is possible by using the (Fowler-Nordheim (F-N)) tunnel phenomenon and the hot electron phenomenon. Electrons are injected in the insulating film around the floating gate with the application of a high electrical field. In the case of hot electron injection, an electric field between a source region and drain region of a MOSFET semiconductor memory device accelerates electrons traveling between the source and drain regions to a velocity where these electrons are termed “hot” electrons. To attract these hot electrons, a floating gate of the FET is biased at a high voltage through a control gate of the FET. As the hot electrons move very fast, their travel time beneath the floating gate of the FET is too short to attract many of the hot electrons into the floating gate of the FET. Thus, electron injection via hot electron injection is not very efficient.